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目录

1.string.h

2.string.cpp

3.test.cpp

4.一些注意点

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本篇博客就学习下如何模拟实现简易版的string类，学好string类后面学习其他容器也会更轻松些。

代码实现如下：

1.string.h

#define  _CRT_SECURE_NO_WARNINGS 1
#pragma once
#include <iostream>
#include <assert.h>
#include <string>
using namespace std;
namespace lwx {class string {private:char* _str=nullptr;size_t _size=0;size_t _capacity=0;public:using iterator = char*;using const_iterator = const char*;string(const char* s = "");string(const string& str);string& operator=(const string& str);~string();size_t size()const {return _size;}size_t capacity()const {return _capacity;}char* c_str()const {return _str;}void clear() {_str[0] = '\0';_size = 0;}string substr(size_t pos, size_t len = npos)const;iterator begin() {return _str;}iterator end() {return _str + _size;}const_iterator begin() const {return _str;}const_iterator end()const {return _str + _size;}string& operator+=(char c);string& operator+=(const char* s);void push_back(char c);void append(const char* s);void erase(size_t pos, size_t len = npos);void insert(size_t pos, char c);void insert(size_t pos, const char* s);size_t find(char c, size_t pos=0)const;size_t find(const char* s, size_t pos=0)const;void reserve(size_t n) {if (n > _capacity) {char* p = new char[n + 1];strcpy(p, _str);delete[] _str;_str = p;_capacity = n;}}char& operator[](size_t n) {assert(n <= _size);return _str[n];}const char& operator[](size_t n)const {assert(n <= _size);return _str[n];}public:static const size_t npos;};bool operator==(const string& st, const string& str);bool operator!=(const string& st, const string& str);bool operator>=(const string& st, const string& str);bool operator<=(const string& st, const string& str);bool operator>(const string& st, const string& str);bool operator<(const string& st, const string& str);ostream& operator<<(ostream& os, const string& str);istream& operator>>(istream& is, string& str);
}

2.string.cpp

#define  _CRT_SECURE_NO_WARNINGS 1
#include"string.h"
namespace lwx {const size_t string::npos = -1;string::string(const char* s):_size(strlen(s)){_capacity = _size;_str = new char[_capacity + 1];strcpy(_str, s);}string::string(const string& str) {_size = str._size;_capacity = str._capacity;_str = new char[_capacity + 1];strcpy(_str, str._str);}string& string::operator=(const string& str) {if (*this != str) {delete[] _str;_size = str._size;_capacity = str._capacity;_str = new char[_capacity+1];strcpy(_str, str._str);}return *this;}string::~string() {delete[] _str;_str = nullptr;_size = 0;_capacity = 0;}void string::push_back(char c) {if (_size == _capacity) {reserve(_capacity==0?4:2*_capacity);}_str[_size] = c;_str[_size + 1] = '\0';_size++;}void string::append(const char* s) {if (_size + strlen(s) > _capacity) {reserve(2 * _capacity);}if (_size + strlen(s) > _capacity)reserve(_size + strlen(s));strcpy(_str+_size, s);_size += strlen(s);}string& string::operator+=(char c) {push_back(c);return *this;}string& string::operator+=(const char* s) {append(s);return *this;}string string::substr(size_t pos,size_t len) const{assert(pos < _size);if (len>_size-pos) {len = _size - pos;}lwx::string sub;sub.reserve(len+1);for (size_t i = 0; i < len; i++) {sub._str[i] = _str[pos + i];}return sub;}void string::erase(size_t pos, size_t len ) {assert(pos <= _size);if (len >= _size - pos) {_str[pos] = '\0';_size = pos;}else {for (size_t i = 0; i <len; i++) {_str[pos+i] = _str[pos + len+i];}}_size -= len;}void string::insert(size_t pos, char c) {assert(pos <= _size);if (_size == _capacity)reserve(_capacity==0?4:2 * _capacity);size_t end=_size+1;while (end > pos) {_str[end] = _str[end - 1];end--;}_str[pos] = c;_size++;}void string::insert(size_t pos, const char* s) {assert(pos <= _size);size_t n = strlen(s);if (_size + n > _capacity)reserve(2 * _capacity);if (_size + n > _capacity)reserve(_size + n);size_t end = _size + n;while (n > 0) {_str[end + n] = _str[end];n--;}for (size_t i = 0; i < n; i++) {_str[pos + i] = s[i];}_size += n;}size_t string::find(char c, size_t pos) const{assert(pos < _size);for (size_t i = pos; i < _size; i++) {if (_str[i] == c)return i;}return npos;}size_t string::find(const char* s, size_t pos) const{assert(pos < _size);const char* p = strstr(_str + pos, s);if (p == nullptr) {return npos;}else {return p - _str;}return npos;}bool operator==(const string& st, const string& str) {return strcmp(st.c_str(), str.c_str()) == 0;}bool operator!=(const string& st, const string& str) {return !(st == str);}bool operator>=(const string& st, const string& str) {return !(st < str);}bool operator<=(const string& st, const string& str) {return !(st > str);}bool operator>(const string& st, const string& str) {return strcmp(st.c_str(), str.c_str()) > 0;}bool operator<(const string& st, const string& str) {return !(st > str || st == str);}ostream& operator<<(ostream& os,const string& str) {for (size_t i = 0; i < str.size(); i++) {os << str[i];}return os;}istream& operator>>(istream& is, string& str) {str.clear();char c;c = is.get();while (c != ' ' && c != '\n') {str += c;c = is.get();}return is;}
}

3.test.cpp

#define  _CRT_SECURE_NO_WARNINGS 1
#include"string.h"void test_string1()
{lwx::string s2;cout << s2.c_str() << endl;lwx::string s1("hello world");cout << s1.c_str() << endl;s1[0] = 'x';cout << s1.c_str() << endl;for (size_t i = 0; i < s1.size(); i++){cout << s1[i] << " ";}cout << endl;// 迭代器 -- 像指针一样的对象lwx::string::iterator it1 = s1.begin();while (it1 != s1.end()){(*it1)--;++it1;}cout << endl;it1 = s1.begin();while (it1 != s1.end()){cout << *it1 << " ";++it1;}cout << endl;// 修改// 底层是迭代器的支持// 意味着支持迭代器就支持范围forfor (auto& ch : s1){ch++;}for (auto ch : s1){cout << ch << " ";}cout << endl;const lwx::string s3("xxxxxxxxx");for (auto& ch : s3){//ch++;cout << ch << " ";}cout << endl;
}void test_string2()
{lwx::string s1("hello world");cout << s1.c_str() << endl;s1 += '#';s1 += "#hello world";cout << s1.c_str() << endl;lwx::string s2("hello world");cout << s2.c_str() << endl;s2.insert(6, 'x');cout << s2.c_str() << endl;s2.insert(0, 'x');cout << s2.c_str() << endl;lwx::string s3("hello world");cout << s3.c_str() << endl;s3.insert(6, "xxx");cout << s3.c_str() << endl;s3.insert(0, "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx");cout << s3.c_str() << endl;
}void test_string3()
{lwx::string s1("hello world");cout << s1.c_str() << endl;s1.erase(6, 2);cout << s1.c_str() << endl;s1.erase(5, 20);cout << s1.c_str() << endl;s1.erase(3);cout << s1.c_str() << endl;
}void test_string4()
{lwx::string s1("hello world");cout << s1.find(' ') << endl;cout << s1.find("wo") << endl;lwx::string s2 = "https://legacy.cplusplus.com/reference/cstring/strstr/?kw=strstr";//bit::string s2 = "https://blog.csdn.net/ww753951/article/details/130427526";size_t pos1 = s2.find(':');size_t pos2 = s2.find('/', pos1 + 3);if (pos1 != string::npos && pos2 != string::npos){lwx::string domain = s2.substr(pos1 + 3, pos2 - (pos1 + 3));cout << domain.c_str() << endl;lwx::string uri = s2.substr(pos2 + 1);cout << uri.c_str() << endl;}
}void test_string5()
{lwx::string s1("hello world");lwx::string s2(s1);cout << s1.c_str() << endl;cout << s2.c_str() << endl;s1[0] = 'x';cout << s1.c_str() << endl;cout << s2.c_str() << endl;lwx::string s3("xxxxxxxxxxxxxxxxxxxxxxxxxxxxx");s1 = s3;cout << s1.c_str() << endl;cout << s3.c_str() << endl;s1 = s1;cout << s1.c_str() << endl;
}void test_string6()
{lwx::string s1("hello world");lwx::string s2(s1);lwx::string s3 = s1;// 构造+拷贝 ->优化直接构造lwx::string s4 = "hello world";cout << (s1 == s2) << endl;cout << (s1 < s2) << endl;cout << (s1 > s2) << endl;cout << (s1 == "hello world") << endl;cout << ("hello world" == s1) << endl;//operator<<(cout, s1); cout << s1 << endl;cin >> s1;cout << s1 << endl;std::string ss1("hello world");cin >> ss1;cout << ss1 << endl;
}int main()
{test_string6();return 0;
}

4.一些注意点

①我们定义的string类会和库里面有冲突的风险，所以我们可以用命名空间namespace进行封装。

②编写默认构造函数时，我们不能给_str(nullptr)缺省值，因为cout<<(const char*)_str<<endl不会当指针打印，它会自动识别类型，觉得你是想打印字符串，而打印字符串遇到'\0'才会终止，但_str为空指针，这就有解引用空指针问题了。

但标准库里的string不会有这种问题，解决方法：直接给'\0'开一个空间就行了

③在string这里申请空间要多给一个空间用来存放'\0',，但capacity不将'\0'计算其中，空间真实大小=_capacity+1。

④在前面类和对象下我们说到，尽可能的使用初始化列表，但在这里使用比较别捏，三个strlen(),strlen是在运行时计算的，3个O(n),还是很坑的。

那我们改一下，下面这个比上面的运行效率时好多了，但是这种写法是错的。因为我们之前说过，初始化列表会按声明的顺序初始化，先走_str,再走_size,所以我们还得把声明顺序变了才行，但声明顺序变了又不顺我们的习惯(先声明指针)，而且过后还要把数据拷贝出来，所以说我们是得尽可能使用初始化列表，但有些东西是初始化列表搞不定的，改该用函数体还是得用，不能说有了初始化列表就不用函数体了。

最终我们可以改成，这样就不用管顺序了。